The following will probably illustrate the mode by which the human mind first acquires the proper use of these general terms. The infant child learns to distinguish one existence from another probably long before he acquires the use of any names by which to designate them. We may suppose that a little dog is an inmate of his nursery, and that with the _sight_ of this animal has often been associated the _sound_ of the word _dog_. This is so often repeated, that, by the principle of association, the sight of the object and the sound of the word invariably recur together. He observes that this sound is used by those around him in order to direct his attention to the animal, and he himself soon uses the word to direct the attention of others in the same way. But soon it happens that another animal is introduced into his apartment, which in many respects resembles the object he has learned to call a dog. To this new object he would apply the same term, but he finds that others use the sound _cat_ in connection with the sight of this new animal. He soon learns the difference between the two objects, the particulars in which they agree, and those in which they differ. He afterward notices other animals of these species, and observes that some have the qualities to which the term _dog_ is applied, and others those to which the term _cat_ is applied. He continues to notice animals of other kinds, and, after long experience in this way, he learns to apply names to designate a particular _combination of qualities_, and, whenever these qualities are found combined, he has a term ready to apply to them. He learns that some words are used to point out the peculiar qualities which distinguish one thing from all others, and, at the same time, other words are used which simply recall _qualities_, but do not designate any particular existence to which they belong. Thus the term _boy_ he uses for the purpose of designating qualities without conceiving of any particular existence in which they are found, while the term _Mary_ is used to designate the qualities and circumstances of the particular existence he finds as the companion of his sports. All objects of our perceptions are arranged into classes, according to the peculiar combination of qualities which are recalled by the names employed to designate them. For example, all objects that have the qualities of existence and of animal life are arranged in one class, and are called _animals_. All those which have the qualities recalled by the term animal, and the additional qualities of wings and feathers, are arranged in another class called _birds_. All those objects which have the qualities included in the term _bird_, together with several additional qualities, are arranged in another class, and called _eagles_.